This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that they should. Mass is conserved.' But there's a problem with that answer. Energy is conserved, but mass isn't; it's turned into another energy store. From what I've learned in school, the amount of mass in the universe is always the same, and that's conservation of mass. But if mass is just another type of energy and can be transferred into other types of energy, mass is certainly not conserved, and the conservation of mass doesn't exist. It's like saying conservation of kinetic energy. Kinetic energy can be transferred into the electrical or thermal or other energy stores, and although the overall energy in the universe is the same, the amount of kinetic energy has changed? Am I being stupid or is my life a lie?